1. Field of the Invention
The present invention relates to a power supply control unit and a power supply control method, and more particularly to a power supply control unit having a semiconductor switch for controlling power supply from a power source to a load by switching control depending on a control signal.
2. Description of the Related Art
FIG. 1 shows a conventional power supply control unit having a semiconductor switch. This conventional power supply control unit supplies a power from a battery to each load selectively and controls the power supply to the load.
In this conventional power supply control unit shown in the same Figure, a shunt resistor RS and a drain D-source S of a thermal FET QF are connected in series in a path for supplying an output voltage VB of a power source 101 to a load 102 like a fuel gauge provided in a fuel tank.
Further, this power supply control unit comprises a driver 901 for detecting a current flowing in the shunt resistor RS so as to control a driving of the thermal FET QF by a hardware circuit, an A/D converter 902 for carrying out ON/OFF control of a driving signal of the thermal FET QF based on a current value monitored by a driver 901, a microcomputer (CPU) 903 and a transistor Q920 for turning on a lamp 921 depending on a control signal from the microcomputer 903.
The thermal FET QF acting as a semiconductor switch contains an over-heat cut-off function for turning itself OFF by an incorporated gate shut-off circuit when an incorporated temperature sensor (not shown) detects that a temperature of the thermal FET QF rises up to a temperature above a predetermined one. A symbol RG in the same Figure indicates a resistor and ZD1 denotes a Zener diode which maintains 12 V between gate G and source S and bypasses when an overvoltage is about to be applied to the gate G.
Further, this conventional power supply control unit also includes a protective function for an overcurrent on the load 102 or between the drain D and source S of the thermal FET QF. Namely, the driver 901 comprises differential amplifiers 911, 913 as a current monitor circuit, a differential amplifier 912 as a current control circuit, a charge pump circuit 915 and a driving circuit 914 for driving the gate G of the thermal FET QF via an internal resistor RG based on an ON/OFF control signal from the microcomputer 903 and a result of overcurrent determination from a current control circuit.
When an overcurrent is detected via the differential amplifier 912 based on a voltage drop in the shunt resistor RS by determining that the current exceeds its predetermined (upper limit) value, the thermal FET QF is turned OFF by the driving circuit 914 and when the current drops below a predetermined (lower limit) value, the thermal FET QF is turned ON.
On the other hand, the microcomputer 903 always monitors a current via current monitor circuits (differential amplifiers 911, 913). When an abnormal current exceeding a normal value flows, it turns OFF the thermal FET QF by turning OFF a driving signal to the thermal FET QF. When the temperature of the thermal FET QF exceeds its predetermined value before the driving signal for OFF control is outputted from the microcomputer 903, the operation of the thermal FET QF is turned OFF by the overheat cut-off function.
The microcomputer 903 determines whether or not a current always monitored via the current monitor circuits (differential amplifiers 911, 913) becomes an abnormal current over its normal value. When such an abnormal current flows, the ON control signal is outputted to the transistor Q920 so as to switch the transistor Q920, thereby supplying power VB to a lamp 921. As a result, it can be notified to an operator, for example, that abnormal current flows to a fuel gauge in the fuel tank.
However, the above-mentioned conventional power supply control unit absolutely needs the shunt resistor RS which is connected directly to a power supply path in order to detect a current. Therefore, because in recent years, current flowing through a load increases enormously with a reduction of the ON resistance on the thermal FET QF, heat loss in the shunt resistor RS cannot be neglected. This is one of problems found in the conventional power supply control unit.
The aforementioned overheat cut-off function and overcurrent control circuit function properly when the load 102 or wiring short-circuits completely, so that a large current flows. However, it does not function properly when a small short-circuit current flows because rare short-circuit like an incomplete short-circuit having some extent of short-circuit resistance occurs. Thus, there is no way but controlling so as to turn OFF the thermal FET QF by detecting the abnormal current by the microcomputer 903 via a monitor circuit for current, so that responsibility of control by microcomputer to abnormal current is worse.
Further, because the shunt resistor RS, microcomputer 903 and the like are necessary, a large installation space is needed. Thus, there is a problem that the cost for the unit is increased by these relatively expensive components.